The Transport Minister and the mayor of Auckland have clashed over a report recommending plans for the city's transport network.

Len Brown issued a statement claiming a study commissioned by Auckland Transport, released today, said a rail link was necessary to stop the transport system from crumbling under the strain of the city's population growth.

The study warns of significant delays and congestion on all routes into the city centre within the next 10 years, said Mr Brown.

Minutes later, Gerry Brownlee responded in a statement, saying a tunnel is not viable.

The City Centre Future Access Study was carried out in an effort to develop an achievable transport programme for access into the city centre.

It identified a city rail link as essential, as bus-only investment would offer only short-term benefits and in some cases would be "worse than doing nothing" for private vehicle travel times, said Mr Brown.

Auckland's population is expected to reach 2.2 million by 2041 and a "significant investment" to complete public transport networks was needed to meet demand, said Mr Brown.

By that year, car travel times into the city centre from some areas are likely to increase by 25 minutes.

"We must complete the City Rail Link (CRL) without delay," Mr Brown said. He believed it was an essential investment for the whole of Auckland because it would double the capacity of the rail network and improve access to other centres including Pukekohe, Manukau and Papakura.

Two potential alternatives to a city rail link were also investigated - a surface bus priority, and an underground bus tunnel.

Neither option performed as well as the CRL, said Mr Brown.

The study also found that private vehicle speeds in the city centre at peak times will more than halve, reducing to seven kilometres per hour (km/h) by 2021 and down to five km/h by 2041.

While the report added to the transport debate, "It also falls some way short of convincing the Government it should provide financial support to any fast tracking of the proposed City Rail Link," said Transport Minister Gerry Brownlee.

The study suggested the CRL is not viable in the next 15 years and that the argument for building it was based on some "extremely optimistic assumptions", he said.

"With a modelled benefit cost ratio of just 44 cents in the dollar, the benefits of the CRL are nowhere near the cost of building it," he said.

Mr Brownlee said he had expected a broader review of possible transport solutions for Auckland.

Changing workplace practices and emerging technology were also likely to have considerable impact on peak hour travel over the next 30 years, said Mr Brownlee.